1. Field of the Invention
This invention relates generally to the field of video display devices which include a cathode ray tube and, more specifically, to those display devices which also include a "yoke" deflection apparatus.
2. Discussion of the Prior Art
A wide variety of video display devices including computer video terminals and the like incorporated a cathode ray tube, or CRT. A CRT includes an electron gun that emits a beam of electrons, which are directed toward a screen. The electron beam is deflected in a controlled manner to scan across the screen. The intensity of the beam is varied to form an image on the screen.
A problem which arises in connection with CRT displays is "image rotation" in which the projected image appears rotated with respect to the horizontal axis of the screen. Image rotation is often caused by the horizontal component of an external magnetic field, one source of which is the earth's magnetic field. The amount of rotation may vary both in direction and magnitude depending on the geographic location of the display, the orientation of the display with respect to the external magnetic field, etc.
One solution to the image rotation problem is to simply disassemble the display device and perform a manual adjustment of the device's deflection apparatus. For example, if the display device includes a yoke magnetic deflection apparatus, the yoke may be manually rotated with respect to the CRT, thereby offsetting the rotation caused by the external magnetic field. However, there are several disadvantages associated with this technique. First, a trained technician is usually required to perform the adjustment and must typically be dispatched to the user's location, which is quite costly for the user.
Second, the disassembling of the display device carries the inherent risk of damage to the interior thereof. In addition, if the display device is later moved to a different geographic location or is simply reoriented with respect to the external magnetic field, a readjustment may be required due to the corresponding change in the direction or magnitude of the external magnetic field.
Another solution to the image rotation problem is to place a shield around the CRT assembly. Shielding is typically quite costly and may create other difficulties. For example, the size of the chassis or housing in which the CRT assembly is disposed must often be increased in order to accommodate the shielding. Moreover, if the CRT assembly requires serice or repair, the shielding obstructs access to the interior of the assembly.
In some CRT displays, an electrostatic deflection apparatus is used to deflect the electron beam, as opposed to a yoke deflector which deflects the beam magnetically. An electrostatic-type deflector includes one or more pairs of plates to which a time-varyin voltage is applied. The electron beam passes between each pair of plates and is deflected by a time-varying electric field that is created by the voltage.
Some electrostatic-type CRT displays, such as an oscilloscope, include a conductive winding which is wrapped directly around the CRT. By providing a current through the winding, a magnetic field is created which may compensate for misalignment of the plates, as well a degradation of the plates due to aging. However, the magnetic field created by the winding cannot be adjusted to compensate for image rotation caused by an external magnetic field.